Reaction
of 1-butene was studied over silicalite-1 and H-ZSM-5
zeolites with different Al contents (SiO2/Al2O3 molar ratio (Si/Al2) = 23, 80, and 280)
to explore an efficient catalyst for the formation of propene as well
as to elucidate the reaction scheme and the relevant acid sites involved
in the reaction. The formation of alkenes, including propene, increased
and those of alkanes and aromatics decreased with decreasing Al content.
The percentage of alkenes other than n-butene isomers
was 60 C-wt % over silicalite-1 at 550 °C with 34.1 C-wt % propene.
Over H-ZSM-5 with Si/Al2 = 23, the formation of alkenes
was negligible, and the main products were alkanes and aromatics,
the sum of alkanes and aromatics being 65.4 C-wt % at 550 °C.
These product distributions are consistently interpreted by the successive
reactions of oligomerization, cracking, and hydrogen transfer. For
oligomerization and cracking, in addition to strong acid sites on
H-ZSM-5 zeolites, weak acid sites present on silicalite-1 act as active
sites. For the hydrogen transfer reaction of alkenes to form alkanes
and aromatics, strong acid sites are required. The scheme can also
be applicable to the reactions of 1-pentene and 1-hexene. The weak
acid sites on silicalite-1 are assumed to be the silanol groups that
act as Brønsted acid above 300 °C. The presence of
strong acid sites on H-ZSM-5 catalysts, which are the OH groups bridging
to Si and Al, results in the consumption of alkenes by hydrogen transfer.
Removal of a part of Al contained in silicalite-1 as an impurity and
enrichment of surface silanol groups on silicalite-1 resulted in the
improvement of the propene yield. It is concluded that silicalite-1
is an efficient catalyst for the formation of propene by the reactions
of light alkenes because of the absence of strong acid sites and the
presence of weak acid sites.
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